The hippocampal engram maps experience but not place

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Science  27 Jul 2018:
Vol. 361, Issue 6400, pp. 392-397
DOI: 10.1126/science.aat5397

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Support for the memory index theory

The link between contextual memory representations and locations or routes represented by hippocampal place cells during exploration remains unknown. Tanaka et al. examined spatial firing properties of neurons in hippocampal area CA1 on the basis of whether they had recently expressed the immediate-early activity-induced gene c-Fos in response to a novel context. The c-Fos–positive neurons displayed a more on-off firing pattern than the c-Fos–negative cells during context discrimination. In a contextual recognition paradigm, these results support the index theory of hippocampal function over a cognitive mapping theory.

Science, this issue p. 392


Episodic memories are encoded by a sparse population of hippocampal neurons. In mice, optogenetic manipulation of this memory engram established that these neurons are indispensable and inducing for memory recall. However, little is known about their in vivo activity or precise role in memory. We found that during memory encoding, only a fraction of CA1 place cells function as engram neurons, distinguished by firing repetitive bursts paced at the theta frequency. During memory recall, these neurons remained highly context specific, yet demonstrated preferential remapping of their place fields. These data demonstrate a dissociation of precise spatial coding and contextual indexing by distinct hippocampal ensembles and suggest that the hippocampal engram serves as an index of memory content.

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